(i) Field of the Invention
The present invention relates to an apparatus for delivering a cryogenic liquid, of the type comprising a thermally insulated main pipe in which a device for throttling the flow of liquid is mounted.
It applies particularly to the controlled delivery of liquid nitrogen to moving containers, for the purpose of inerting them and/or of pressurizing them.
(ii) Description of the Related Art
As is well known, the inevitable heat influx into pipes conveying cryogenic liquids causes the appearance of bubbles in these liquids, making it difficult to transfer them at a constant flow rate, especially for flow rates of less than 10 l/h.
One situation particularly sensitive to this phenomenon is encountered in the process of pressurizing cans using liquid nitrogen. In this technique, a row of cans in contact with each other, after they have been filled with a noncarbonated product, for example a still drink, move along beneath a nozzle from which a thin stream of liquid nitrogen flows. Each can thus receive a few drops of liquid nitrogen and is then hermetically sealed.
The start of vaporization of the nitrogen, before the can is sealed, expels the air from the can, thus ensuring that the product is inerted. Next, after the can has been sealed, the end of vaporization of the nitrogen pressurizes the can, thereby allowing it to be handled as desired without the risk of it being dented or crushed.
As will have been understood, if the quantity of liquid nitrogen received by a can is too low, the pressurization will be insufficient to maintain the integrity of the can, and if this quantity is too great there is a risk of the can exploding.
Current filling systems make several tens of cans per second move along in a single line, and increasingly high rates are envisaged. Furthermore, if one takes into account the periods of acceleration and braking of the systems during the start and stop phases of the plant, it will be understood that a very precise control of the liquid nitrogen flow rate dispensed at each instant is critical for the development of the process.